1. Field of the Invention
This invention pertains generally to rectifier circuits and, more particularly, to three-phase semiconductor controlled rectifier (SCR) bridges for converting alternating current (AC) to direct current (DC).
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC. These circuits commonly use SCRs disposed in bridge segments, with typically one SCR for each polarity of each AC phase. The firing point for each rectifier in the cycle of the AC waveform is normally controlled by a bridge firing control circuit.
It is not uncommon for a number of bridges to be operated in parallel with each of the bridge firing control circuits being controlled by a central firing control circuit. The central firing control circuit manages each of the bridge firing control circuits in order that the corresponding rectifiers in each of the parallel bridges conduct current at the exact same point in the AC waveform.
It is desirable to have each of the SCRs in each corresponding segment of the several bridges fired at the exact same time to ensure the quality of the DC signal output and avoid the necessity for extensive filtering circuits. Unfortunately, even slight variations in the nature of the components can vary the resistance and inductance of the bridge segments, thereby affecting their outputs and resulting in some imbalance. This imbalance can further affect the heating of the various components in the different bridge segments and, thus, also affect the outputs.
Typically, average current values for each bridge segment are measured and employed by the control firing control circuit to balance the outputs of the parallel bridges. In prior art digital systems, where individual currents for each SCR are measured at specific times, it is possible to calculate any function of the current that each cell or segment carries (e. g., the average current) as determined from instantaneous currents as sensed by corresponding individual sensors for each of the respective bridge segments.
Accordingly, there is room for improvement in terms of an improved rectifier bridge and system for measuring average current per bridge segment.